The invention relates to a detection device for the imaging of an object. The invention relates also to a detection method for the imaging of an object.
Detection devices and methods are known in the state of the art for the imaging of an object. Among detection devices for the imaging of an object, a distinction is made between far field detection devices and near field detection devices. It is known that the advantage of near field detection devices is to allow observation of objects of small dimensions, notably smaller than the size defined by the Rayleigh criterion.
The invention more particularly relates to a detection device for the imaging of an object in a near field. It relates to a detection device for the imaging of an object, comprising:                a laser cavity adapted for emitting an original light signal at an original wavelength towards the object so as to generate an evanescent wave at the surface of the object;        conversion means adapted for transforming the evanescent wave into a progressive signal having an output wavelength;        re-injection means adapted for injecting the progressive signal into the laser cavity so as to generate interferences inside the laser cavity between the progressive signal and the original light signal;        detection means adapted for detecting the interferences in order to determine at least one physical characteristic of the object.        
Such a device is known from document FR-A-2785045. In this document, the re-injection into the laser cavity allows the cavity to be used as an interferometer, and the interferences generated in the cavity are used for measuring an amplitude of evanescent waves and therefore a measurement of the characteristics of the object may be obtained. In this document, detection of the interferences is a homodyne detection with which the amplitude of the evanescent waves may be determined. However, with such detection, it is not possible to obtain information on the phase of the evanescent wave. Consequently, certain characteristics of the object cannot be measured by the device described in the aforementioned document.
The problem solved by the invention is to improve the measurement of the evanescent wave generated at the surface of the object in a detection device for the imaging of an object. In particular, the problem solved by the invention is to be able to measure the phase of the evanescent wave generated at the surface of the object in a detection device for the imaging of an object. This problem is solved by the fact that the detection device for the imaging of an object described above comprises wavelength modification means adapted so that the wavelength of the progressive signal injected into the laser cavity is different from the original wavelength.
By these wavelength modification means, two signals with different wavelengths propagate in the laser cavity, which causes the generation of dynamic interferences in the form of heterodyne beats. Consequently, heterodyne detection of these beats is feasible by detection means. Unlike the re-injection device described earlier, the invention therefore allows access to the phase of the evanescent wave generated by the interaction between the original signal and the progressive signal from the object. The measurement of the characteristics of the object is therefore enhanced by the invention.
According to the invention, by generating heterodyne beats, it is possible to obtain better contrast upon detecting the amplitude and phase of the evanescent wave. It therefore allows measurements on the evanescent wave, even if the collected intensity of the evanescent wave is low. Still in this embodiment, the laser cavity may be capable of generating relaxation oscillations and, in this case, the wavelength modification means are adapted so that the difference between the output wavelength and the original wavelength allows these relaxation oscillations to be excited.
In this embodiment, when the laser cavity is capable of generating relaxation oscillations, notably when the laser is a laser of class B, the beat caused by the progressive signal injected into the laser cavity enters resonance with the relaxation oscillations, which allows a significant gain to be obtained on the detection of the amplitude and of the phase. This gain depends on the characteristics of the laser cavity of class B, and it may be of the order of one million notably for a laser cavity of the solid microlaser type. Still in this embodiment, the laser cavity may be adapted for emitting the original light signal in the infrared. The generation of the original light signal in the infrared has the advantage of allowing the use of highly developed standard telecommunication components. In particular, fiber optic components are current in this range of wavelengths. Further, this wavelength range is difficult to access with conventional interferometric setups.
According to an embodiment of the invention, the wavelength modification means comprise at least one acousto-optical modulator. In this case, the or each acousto-optical modulator has a preferential shift frequency and the combination of these accumulated shifts allows adjustment on the resulting shift for exciting the relaxation oscillations of the re-injected laser. According to an embodiment, the device may comprise a first optical isolator positioned in the path of the original light signal so as to avoid propagation of a parasitic reflected optical signal towards the laser cavity. This has the advantage of not interfering with the laser cavity in addition to the re-injection achieved by the device described above. In particular, this avoids reflections by lenses, by acousto-optical modulators or the object. In this case, the or each acousto-optical modulator is positioned after the first optical isolator.
Preferably, the device further comprises a second optical isolator positioned in the path of the progressive signal. In this case, the or each acousto-optical modulator may be positioned between the first optical isolator and the second optical isolator. According to an embodiment, the conversion means comprise a microtip. The invention also relates to a microscope comprising a detection device for the imaging of an object as described earlier.
The invention also relates to a detection method for the imaging of an object comprising steps in which:                a laser cavity emits an original light signal at an original wavelength towards the object so as to generate an evanescent wave at the surface of the object;        the evanescent wave is converted into a progressive signal;        the progressive signal is injected into the laser cavity so as to generate interferences within the laser cavity between the progressive signal and the original light signal;        the interferences are detected so as to determine characteristics of the object,wherein the wavelength of the progressive signal injected into the laser cavity is different from the original wavelength.        